The present invention relates generally to apparatus for electrotreating a metal strip and more particularly to electrotreating apparatus with a depletable anode roll.
An example of electrotreating is the electrolytic plating of a surface of a metal strip, e.g., electrolytic galvanizing wherein a steel strip is plated with zinc. Other examples of electrotreating include the electrolytic cleaning or pickling of a surface of the metal strip. In all these examples the metal strip is electrically charged and constitutes one electrode in an electrolytic cell having another electrode with electrolytic liquid between the two electrodes. An electric current flows through the electrolytic liquid between the metal strip and the other electrode, and, depending upon whether the metal strip is to be plated or cleaned, ions flow to or from a strip surface to be either deposited thereon or removed therefrom.
For example, in an electrogalvanizing operation, the metal strip is provided with a negative charge, so as to be a cathode, a metallic anode is placed adjacent the metal strip surface to be coated, and the electrolytic liquid contains zinc ions. The anode may be depletable, in which case it is composed of zinc.
It is often desirable to coat only one surface of the metal strip with zinc, and in such a case, a zinc anode is placed alongside only that surface of the metal strip which is to be coated.
A recently developed electroplating process employs a horizontally disposed, cylindrical, depletable anode roll comprising an outer layer composed of the plating metal (e.g., zinc) and having a peripheral surface. The depletable anode roll is located totally above a bath of electrolytic liquid, and the roll itself is not in contact with the bath. A continuous metal strip having opposed flat surfaces is wrapped around a substantial portion of the anode roll, with an inner surface of the strip in closely spaced relation to the conductive, peripheral surface of the anode roll. The metal strip is advanced in a downstream direction, and the roll is simultaneously rotated while maintaining the wrapped-around, spaced relationship between the strip and the roll.
As the strip advances in a downstream direction, the inner surface of the wrapped-around portion of the strip is electroplated, from one side edge of the strip to the other side edge thereof, without electroplating the other surface of the strip. This is accomplished by maintaining an electrolytic liquid in the space between the anode roll's peripheral surface and the inner surface of the wrapped-around portion of the strip, from one side edge of the strip to the other side edge thereof, while, at the same time, maintaining the outer surface of the strip out of contact with the electrolytic liquid.
The arrangement described in the preceding sentence is accomplished by surrounding the peripheral surface of the anode roll with a concentric layer of mesh composed of electrically insulative material which prevents direct electrical contact between the strip and the anode roll's peripheral surface. The anode roll's peripheral surface and the mesh together define a multiplicity of open-end electrotreating sites each having an inner base defined by a part of the roll's peripheral surface, site-enclosing side walls defined by a part of the mesh, and an open outer end opposite the base. Wrapping the continuous metal strip around a portion of the roll's peripheral surface closes the sites on the wrapped portion of the anode roll. As the strip advances in a downstream direction and the anode roll rotates, those sites which were previously covered become uncovered, and sites on a portion of the roll previously not wrapped by the strip become covered.
An electrolytic liquid is introduced onto the peripheral surface of the anode roll at a location at or in advance of the location where the metal strip joins the roll, and this floods with liquid the sites closed by the strip as the strip advances and the roll rotates. This creates an electrolytic cell at each closed site wherein that portion of the roll's peripheral surface defining the site's inner base is the anode, that portion of the strip's inner surface closing that site is the cathode and the electrolytic liquid confined within the closed site is the electrolytic bath.
Because the inner surface of the strip is in contact with the electrolytic liquid, while the outer surface of the strip is not, the deposition of cation on the outer surface of the strip is effectively prevented.
The electroplating process and apparatus described above is disclosed in more detail in the prior filed, commonly-owned U.S. application Ser. No. 424,858 filed Sept. 27, 1982, entitled "Method and Apparatus for Electro-Treating a Metal Strip," William A. Carter inventor; and the disclosure thereof is incorporated herein by reference.
A problem arises when a continuous metal strip is electroplated with a depletable anode roll covered by a concentric layer of mesh composed of electrically insulative material. The concentric mesh layer overlies the entirety of the anode roll's peripheral surface and is essentially stationary thereon. The mesh is typically composed of criss-crossing strands of electrically insulative material defining either a regular grid pattern or an irregular pattern. During an electroplating operation, the depletable anode roll erodes at those areas of its peripheral surface not covered by the strands of electrically insulative mesh material, but the anode roll's peripheral surface does not erode where the mesh strands contact the roll.
Because there is selective erosion of those areas of the roll's peripheral surface not covered by the mesh strands, while those areas of the peripheral surface covered by the mesh strands do not erode, cratering occurs on the roll's peripheral surface. The craters become deeper and deeper and the ridges between craters become relatively higher and higher as the electroplating operation proceeds. This is wasteful with regard to the plating metal because those parts of the depletable anode rolls forming the crater rims are not utilized in the plating operation. In addition, cratering causes premature destruction of the mesh layer, requiring frequent replacement thereof.
The aforementioned problems are indigenous to a depletable anode roll and can be avoided by utilizing a non-depletable anode roll composed of some metal other than that which is to be plated on the metal strip. For example, in an electrogalvanizing operation, the anode roll may comprise lead as the outer layer. In such a case, the cation of zinc to be plated out on the metal strip through the medium of the electrolytic liquid must be continuously supplied to the liquid from some source other than the anode roll. Such an arrangement is not as convenient as supplying the zinc cation from a depletable zinc anode roll. In addition, there are other advantages to employing a depletable zinc anode roll versus a non-depletable lead anode roll.
For one thing, a zinc anode requires less power to perform the electroplating process than does a lead anode, and a zinc anode generates less heat than does a lead anode so that the electrolytic liquid needs substantially less cooling. In addition, when the anode roll's outer layer is composed of lead, the electrolytic liquid must be a zinc sulfate solution; however, when the anode roll's outer layer is composed of zinc, the electrolytic liquid may be either a zinc sulfate solution or a zinc chloride solution. Zinc chloride solution is a better conductor than zinc sulfate solution, thereby decreasing the resistance and increasing the current density. Zinc chloride solution cannot be used when the anode roll's outer layer is composed of lead because a chloride skin will form on the lead anode and, in effect, kill the lead anode electrically. The chloride skin has a high electrical resistance.
Moreover, when the anode roll's outer layer is composed of lead, the H.sub.2 O in the dilute acid solution contained in the electrolytic liquid is broken down at the lead anode into hydrogen plus oxygen, and the oxygen takes up space in the sites which contain the electrolytic liquid. This reduces the amount of electrolytic liquid which can be contained in a site, which is undesirable. This is not a problem, however, when employing a zinc anode and zinc chloride solution.